feat(sdk): add definePageLayoutTab for extending existing page layouts (#20004)

## Summary

Introduces `definePageLayoutTab` so apps can attach a single tab (with
optional widgets) to an **existing** `pageLayout` referenced by
`pageLayoutUniversalIdentifier`. The parent layout can be standard, from
the same app, or from another app — mirroring how `defineField`
references an object via `objectUniversalIdentifier`.

This complements `definePageLayout`: use `definePageLayout` when you own
the entire layout, use `definePageLayoutTab` when you only want to add
to one.

```ts
import { definePageLayoutTab, PageLayoutTabLayoutMode } from 'twenty-sdk/define';

export default definePageLayoutTab({
  universalIdentifier: 'b1b2b3b4-b5b6-4000-8000-000000000001',
  pageLayoutUniversalIdentifier: 'STANDARD-OR-OTHER-APP-PAGE-LAYOUT-UUID',
  title: 'Hello World',
  position: 1000,
  icon: 'IconWorld',
  layoutMode: PageLayoutTabLayoutMode.CANVAS,
  widgets: [/* ... */],
});
```

## Changes

- **twenty-shared**: new top-level `pageLayoutTabs:
PageLayoutTabManifest[]` on `Manifest`, optional
`pageLayoutUniversalIdentifier` on `PageLayoutTabManifest`, new
`SyncableEntity.PageLayoutTab`.
- **twenty-sdk**:
  - new `definePageLayoutTab` + `PageLayoutTabConfig` exports;
- manifest extraction wiring (`TargetFunction.DefinePageLayoutTab`,
`ManifestEntityKey.PageLayoutTabs`);
  - dev-mode label/state for the new entity;
- CLI scaffold (`getPageLayoutTabBaseFile`) + unit tests for `npx
twenty-cli add`.
- **twenty-server**: convert top-level `pageLayoutTabs` (and their
widgets) into universal flat entities in
`computeApplicationManifestAllUniversalFlatEntityMaps`. Cross-app FK
validation on `pageLayoutUniversalIdentifier` is already handled by the
existing `FlatPageLayoutTab` validator.
- **docs**: new `definePageLayoutTab` accordion in `apps/layout.mdx`
with usage example and guidance vs `definePageLayout`.
- **CI / rich-app fixture**: `extra-tab.page-layout-tab.ts` exercises
the new flow with a front-component widget; `expected-manifest.ts` and
`manifest.tests.ts` updated.

packages/twenty-sdk/src/cli/__tests__/apps/minimal-app/__integration__/app-dev/expected-manifest.ts

@@ -315,6 +315,7 @@ export const EXPECTED_MANIFEST: Manifest = {
   views: [],
   navigationMenuItems: [],
   pageLayouts: [],
+  pageLayoutTabs: [],
   roles: [
     {
       universalIdentifier: 'e1e2e3e4-e5e6-4000-8000-000000000040',

packages/twenty-sdk/src/cli/__tests__/apps/rich-app/__integration__/app-dev/expected-manifest.ts

@@ -4,6 +4,7 @@ import { PermissionFlagType } from 'twenty-shared/constants';
 import {
   FieldMetadataType,
   NavigationMenuItemType,
+  PageLayoutTabLayoutMode,
   RelationOnDeleteAction,
   RelationType,
   ViewCalendarLayout,
@@ -12,6 +13,28 @@ import {
 
 export const EXPECTED_MANIFEST: Manifest = {
   pageLayouts: [],
+  pageLayoutTabs: [
+    {
+      universalIdentifier: 'b0b1b2b3-b4b5-4000-8000-000000000010',
+      pageLayoutUniversalIdentifier: 'b0b1b2b3-b4b5-4000-8000-000000000020',
+      title: 'Extra Tab',
+      position: 1000,
+      icon: 'IconLayout',
+      layoutMode: PageLayoutTabLayoutMode.CANVAS,
+      widgets: [
+        {
+          universalIdentifier: 'b0b1b2b3-b4b5-4000-8000-000000000011',
+          title: 'Extra Widget',
+          type: 'FRONT_COMPONENT',
+          configuration: {
+            configurationType: 'FRONT_COMPONENT',
+            frontComponentUniversalIdentifier:
+              '370ae182-743f-4ecb-b625-7ac48e21f0e5',
+          },
+        },
+      ],
+    },
+  ],
   publicAssets: [
     {
       checksum: '99496069dcc2a1488e1cae9f826d2707',

packages/twenty-sdk/src/cli/__tests__/apps/rich-app/__integration__/app-dev/tests/manifest.tests.ts

@@ -26,6 +26,7 @@ export const defineManifestTests = (appPath: string): void => {
       expect(manifest.fields).toHaveLength(23);
       expect(manifest.views).toHaveLength(5);
       expect(manifest.navigationMenuItems).toHaveLength(3);
+      expect(manifest.pageLayoutTabs).toHaveLength(1);
 
       expect(normalizeManifestForComparison(manifest)).toEqual(
         normalizeManifestForComparison(EXPECTED_MANIFEST),

packages/twenty-sdk/src/cli/__tests__/integration/utils/normalize-manifest.util.ts

@@ -31,6 +31,7 @@ export const normalizeManifestForComparison = <T extends Manifest>(
   views: sortById(manifest.views),
   navigationMenuItems: sortById(manifest.navigationMenuItems),
   pageLayouts: sortById(manifest.pageLayouts),
+  pageLayoutTabs: sortById(manifest.pageLayoutTabs ?? []),
   logicFunctions: sortById(
     manifest.logicFunctions?.map((fn) => ({
       ...fn,